1. Field of the Invention
The present invention relates generally to satellite and other communication systems subject to severe path loss, and more specifically, to a method of providing paging signals, referred to as deep paging, which results in paging signals that are receivable in the presence of high levels of attenuation.
2. Related Art
Conventional satellite-based communication systems include gateways, user terminals, and one or more satellites to relay communication signals between the gateways and the user terminals. A gateway is an earth station having an antenna for transmitting signals to and receiving signals from satellites. A gateway provides communication links, using satellites, for connecting a user terminal to other user terminals or users of other communication systems, such as a public switched telephone network. A user terminal is a wireless communication device such as, but not limited to, a cellular or satellite telephone, a data transceiver, and a paging receiver. A user terminal can be fixed, portable, or mobile, such as a mobile telephone. A satellite is an orbiting receiver, repeater, and regenerator used to relay information.
A satellite can receive signals from and transmit signals to a user terminal provided the user terminal is within the xe2x80x9cfootprintxe2x80x9d of the satellite. The footprint of a satellite is the geographic region on the surface of the Earth within the range of signals of the satellite. The footprint is usually geographically divided into xe2x80x9cbeams,xe2x80x9d through the use of beam-forming antennas. Each beam covers a particular geographic region within the footprint. Beams may be directed so that more than one beam from the same satellite covers the same specific geographic region.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, entitled xe2x80x9cMethod and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,xe2x80x9d both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, such as data or traffic, to and from a gateway. Specifically, communication signals originating at the gateway are transmitted to a user terminal over a xe2x80x9cforward communication link,xe2x80x9d whereas, communication signals originating at a user terminal are transmitted to the gateway over a xe2x80x9creverse communication link.xe2x80x9d
On the forward communication link, information is transmitted from a gateway to a user terminal over one or more beams. These beams often comprise a number of so-called subbeams (also referred to as frequency division multiple access (FDMA) channels, or in the case of spread spectrum CDMA channels) covering a common geographic area, each occupying a different frequency band. More specifically, in a conventional spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or xe2x80x9cspreadxe2x80x9d user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. On the forward link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different gateways or over different beams, as well as between multipath signals. These codes are often shared by all communication signals within a given subbeam.
In a conventional CDMA spread-spectrum communication system, xe2x80x9cchannelizingxe2x80x9d codes are used to form multiple channels within a satellite sub-beam on a forward link. The channelizing codes are unique xe2x80x98coveringxe2x80x99 or xe2x80x98channelizingxe2x80x99 orthogonal codes that create orthogonal channels in a subbeam over which communication signals are transferred. Walsh functions are generally used to implement the channelizing codes, also known as Walsh codes or Walsh sequences, and create what are known as Walsh channels. A typical orthogonal code length is 64 code chips for terrestrial systems and 128 code chips for satellite systems.
A majority of the orthogonal channels are traffic channels that provide messaging between a user terminal and a gateway. The remaining channels often include a pilot channel, a sync channel, and one or more paging channels. Signals sent over the traffic channels are generally intended for reception by one user terminal, although messages can also be broadcast to multiple users. In contrast, paging, sync, and pilot channels are generally monitored by multiple user terminals.
When a user terminal is not involved in a communications session (that is, the user terminal is not receiving or transmitting traffic signals), the gateway can convey information to the user terminal by transmitting a page to the user terminal. The page, which is usually a short message, is transmitted over the above mentioned paging channel. Pages are often sent by the gateway to establish a communication link with a user terminal, to notify a user terminal that it is being called, to reply to a user terminal trying to access the system, and for user terminal registration. Pages are also used to distribute traffic channel assignments and system overhead information to user terminals. Pages transmitted over the paging channel typically have a data rate on the order of 9600 or 4800 bits per second.
Unfortunately, a user terminal typically encounters problems receiving pages when the user terminal is inside a building or there is some structure or other obstruction positioned between the user terminal and the satellite (such as a tree, geological object, or a building). In such a situation, the user terminal is unable to acquire a page, paging message, or paging signal because the page is unable to penetrate the building or other material due to a propagation loss of the signal as it propagates through the structure. An obvious solution to overcome the propagation loss is to increase the power of the paging channel. The problem with this approach is that in order to overcome such signal attenuation or blockage the power of the paging channel has to be increased significantly. Generally, this requires increasing the signal strength to such a level that a power flux density (PFD) limit would be exceeded in the surrounding area. That is, governmental licensing restrictions and technical constraints place limits as to the allowed amount of power flux density satellite signals can have over a given area or geographical region. Increasing the power to successfully page a blocked or impeded user terminal, places the surrounding area over the allowed power flux density (PFD).
What is needed, therefore, is a method that can provide what will be called xe2x80x9cdeep pagingxe2x80x9d without increasing the power of the paging signal. Deep paging refers to being able to page a user terminal in an environment where there is an excess propagation loss to be overcome, a loss much higher than normally encountered, which is typically on the order of 20 or 30 dB. Such an environment includes a situation where a user terminal is located deeply inside a building or structure, or behind a partial obstruction.
The present invention provides a method for deep or highly penetrative paging that does not require increasing the power of the paging signal. In one aspect, the invention provides a method for deep paging that includes the steps of generating a paging channel message, covering or modulating the paging channel message with an orthogonal code sequence, preferably a Walsh code sequence, having a length greater than or equal to 2m code chips, where m is the length of code sequences typically used for generating traffic channels, and transmitting the modulated paging channel message at a data rate of less than 4800 bits per second (bps). By transmitting the paging channel message at a low data rate, the message is able to penetrate buildings and other structures, thereby allowing a user terminal that is inside a building to be successfully paged. Preferably, the paging channel message is covered by a Walsh sequence of length 65536, the data rate is less than 10 bps, and the Walsh sequence is an auxiliary Walsh sequence.
The invention also provides a method for deep paging in a CDMA communication system where several orthogonal or Walsh code sequences are used to form several orthogonal channels. The method includes the steps of creating at least one auxiliary Walsh sequence from one of the plurality of Walsh sequences, generating a paging channel message, covering or channelizing the paging channel message with the auxiliary Walsh sequence, spreading the channelized paging channel message, and transmitting the spread paging channel message at a data rate of less than 4800 bps. The auxiliary Walsh sequence has a length greater than or equal to 128 chips, but preferably the auxiliary Walsh sequence is of length 65536 chips. Further, preferably, the data rate is less than 10 bps. Advantageously, the method further includes the step of creating a second auxiliary Walsh sequence from one of the plurality of Walsh sequences and covering or modulating a synchronization signal with the second auxiliary Walsh sequence, to create an auxiliary synchronization channel. Additional auxiliary Walsh sequences can be created from one or more of the Walsh sequences and used to cover or modulate additional pilot, synchronization, or paging signals.
The invention also provides a method for compensating for the Doppler effect in a communication system where messages are transmitted at a low data rate to a user terminal that is inside a building or otherwise obstructed. The method includes the steps of having the user terminal acquire a pilot signal prior to the user terminal entering the building, placing the user terminal into a deep paging mode after the user terminal has acquired the pilot signal, proceeding into the building with the user terminal, tracking Doppler as the user terminal proceeds into the building, enter a longer signal integration mode, and monitoring an auxiliary paging channel after activating the deep paging mode. Paging channel messages transmitted over the auxiliary paging channel are modulated by a Walsh sequence having a length greater than or equal to 2m chips, where m is the length of the code used to normally modulate traffic channel signals, and are transmitted at a data rate of less than 4800 bps. Preferably, the paging channel messages transmitted over the auxiliary paging channel are formed using a Walsh sequence having a length of about 65536, and are transmitted at a data rate of 10 bps or less.
The invention also provides an alternative method for compensating for the Doppler effect in a communication system where messages are transmitted at a low data rate to a user terminal that is inside a building or otherwise obstructed. The alternative method includes the steps of receiving ephemeris messages transmitted from a gateway at the user terminal, storing the ephemeris messages or data contained therein in the user terminal, determining the location of the user terminal, determining Doppler based on the determined location of the user terminal and the ephemeris messages stored in the user terminal, and acquiring a pilot signal. In a first embodiment, the step of determining the location of the user terminal includes the steps of storing the location of the user terminal each time the user terminal registers with a gateway and determining the current location of the user terminal based on the location of the user terminal at the time the user terminal last registered with a gateway. In a second embodiment, the step of determining the location of the user terminal includes the steps of receiving a global positioning system (GPS), or other position location system, signal and determining the location of the user terminal based on the GPS signal.